1. Field of the Invention
Briefly, this invention concerns seismic exploration in general. More specifically, it deals with a method that concerns the specialized type of seismic exploration which employs a vibrator-type of generator to create the seismic energy.
2. Description of the Prior Art
The invention relates to a method of suppressing noise trains occurring in a vibrator-type seismogram due to harmonics of the vibrator-type signal which, for instance, may be caused by non-linear effects in the hydraulic of the vibrator and/or by the non-linear reaction of the ground to the oscillation of the vibrator plate.
In seismic exploration operations which use vibrators, there are relatively long signals generated. These long signals (hereinafter called "sweeps") are usually several seconds in duration, of constant amplitude, and with monotonously increasing or decreasing frequency. The technical terms for describing which way the frequency is changing, are "unsweep" and "downsweep" respectively. Such long signals are transmitted into the subsurface by means of one or more vibrators. The signals reflected from changes in the elastic properties connected with the boundaries of geological formations, are recorded by means of geophones located on the earth's surface.
A whole series of identical sweep signals (16, for example) are emitted one after the other to improve the signal/noise ratio. The elementary vibrator-type seismograms (hereinafter called elementary seismograms) are individually recorded. Then they are added giving the final vibrator-type seismogram. However, because the signals are several seconds long the reflections from successive interfaces of geological formations overlap. Consequently, in order to interpret the vibrator-type seismogram in the same manner as a conventional dynamite seismogram where the signal is a short pulse, the long sweep signals occurring in the vibrator type seismogram must be compressed to short signals. This is done by correlating the final vibrator-type seismogram with the sweep signal.
Because of the non-linear reaction of the ground to the vibration of the vibrator base plate and/or because of the non-linearity in said vibrator drive, the emitted signal is not only the proper sweep signal but also contains its harmonics. In correlating said vibrator-type seismogram these harmonics lead to asymmetrical secondary lobes which occur with upsweeps before the main pulse (precursor or forerunner) and with downsweeps after the main pulse (tail). The n-th harmonic of a T-seconds long downsweep linearly decreasing from the frequency f .sub.o to the frequency f .sub.u leads to tails that arrive from t .sub.1 to t .sub.2 seconds after the main pulse, EQU t.sub.1 = (n-1) Tf.sub.u /(f.sub.o - f.sub.u) EQU t.sub.2 = (n-1) Tf.sub.o /[ n(f.sub.o - f.sub.u)]
Therefore, f.sub.o must be greater than nf .sub.u. Hence with a 60/12 Hz downsweep, tails are possible due to harmonics up to the fourth order.
The tails occurring with downsweeps are particularly disturbing since they interfer with later occurring weaker reflections whereas the precursors connected with upsweeps are less disturbing as they interfer with earlier, usually stronger, reflections. The advantage of the downsweeps is to be seen in that they are less demanding on the vibrators, particularly at lower frequencies, and that synchronous operation of the vibrators at low frequencies is considerably easier to achieve.
In order to suppress the even harmonics in said sweep signal, Sorkin (U.S. Pat. No. 3,786,409) suggested a method described as "inverted stacking" the essential feature of which is that only one half of the elementary seismograms are generated with the proper sweep. The other half of the elementary seismograms are generated with a sweep which has the opposite polarity, and simultaneously the polarity of the seismic data at the input of the amplifying/recording system is reversed. The elementary seismograms resultant therefrom are added up as usual (vertically stacked).
That prior method suppressed or at least heavily reduced only the even harmonics. On the other hand the method according to this invention suppresses odd harmonics also.
Consequently, it is an object of this invention to provide a method wherein with regard to the aforementioned type of seismic exploration method, harmonics of the emitted sweep signal may be suppressed up to any order desired.